You do or do not have the Homebrew Channel installed? If you do not have it installed, you should be able to run the Hackmii Installer through Priiloader if you are unable to get LetterBomb to work. However, it should be noted that there is no reason why LetterBomb should not be working unless something is being done incorrectly. Anyway, enter the Hackmii Installer with Priiloader's install file option.

What version of Priiloader do you have installed? If it is the latest, download the hacks_hash.ini file, put it in the Priiloader folder in your apps folder, go into Priiloader, and enable the hack that re-enables Bannerbomb. Once completed, follow the normal instructions for Bannerbomb as if you were on System Menu 4.2 to install the Homebrew Channel.